The present invention relates to a method and a device for driving a solenoid valve.
Electrically controlled solenoid valves are used as pressure control valves in many areas of technology. One particular area of the use of solenoid valves of this type is the braking system of a motor vehicle, in which the pressure in the wheel brake cylinders is controlled by actuating solenoid valves. One example of this type of control system of a solenoid valve is described in German Published Patent Application No. 195 01 760 (U.S. Pat. No. 5,727,852). There, a hydraulic braking system is described in which the pressures to be controlled in the wheel brake cylinders are adjusted by controlling at least one solenoid valve in accordance with a setpoint value. By actuating this solenoid valve in at least one wheel brake, pressure is built up, maintained, or reduced. In this context, the solenoid valve is controlled in the context of a pressure-regulating circuit, which sets a preestablished setpoint value for the pressure in the wheel brake.
A further method and a further device for controlling a pressure-regulating valve in a braking system is described in German Published Patent Application No. 196 54 427. There, for controlling the pressure in at least one wheel brake in the context of a pressure-regulating circuit, at least one control valve is driven such that the valve demonstrates offset behavior. In this context, a pressure regulator is provided which generates a pulse-width-modulated driving signal for the valve or a current setpoint value for the current flowing through the valve winding as a function of the deviation of the pressure measured in the area of a wheel brake from a preestablished setpoint value. The opening cross-section of the valve and therefore the volumetric flow through the valve is adjusted in accordance with the driving signal quantity. As a result of the feedback control, a predetermined pressure differential is set across the valve. The pulse-width-modulated driving signal, in this context, is selected such that an average current through the valve winding is set corresponding to the opening cross-section.
In some application cases, setting a very small pressure differential using a valve drive of this type has proven to be problematic because under certain operating conditions a spontaneous opening of the solenoid valve has been observed.
It is an objective of the present invention to indicate measures for driving a solenoid valve, which can function to set even a very small pressure differential across the valve.
Setting a very small pressure differential is made possible even in valves that, at least under certain operating conditions, open spontaneously in response to small pressure differentials. As a result of the fact that, using the driving method, it is not an average current that is set but rather, in the context of a preestablished frequency, a current that brings about an alternating closing and opening of the valve for setting the small pressure differential, the spontaneous opening of the valve is counteracted. Even in the event that the solenoid valve spontaneously opens, the valve is closed once again by an increased flow of current brought about in accordance with the stipulation of the drive frequency, and in this manner the desired pressure differential is maintained.
In an advantageous manner, the driving process is selected such that the opening ranges of the valve (stroke ranges) are avoided, in which a spontaneous opening of the valve is expected, or the dwell time of the solenoid valve is limited in these ranges, due to the fact that the valve is always actuated by the drive outside of these stroke ranges.
Particularly advantageous is the application of the driving method to a solenoid valve which is used in connection with the electrical control of a braking system, in particular with buildup of very small brake pressure values, such as is desirable, e.g., in connection with an automatic braking process.